Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, composite, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions. III-nitride devices are often formed as inverted or flip chip devices, where both the n- and p-contacts formed on the same side of the semiconductor structure, and light is extracted from the side of the semiconductor structure opposite the contacts.
Silver is often used as a reflective p-contact and is known to be susceptible to transport induced by mechanical stress, chemical reaction, or electromigration. For example, a III-nitride LED with a silver p-contact is illustrated in FIG. 1 and described in U.S. Pat. No. 6,946,685. U.S. Pat. No. 6,946,685 teaches “silver electrode metallization is subject to electrochemical migration in the presence of moisture and an electric field, such as, for example, the field developed as a result of applying an operating voltage at the contacts of the device. Electrochemical migration of the silver metallization to the pn junction of the device results in an alternate shunt path across the junction, which degrades efficiency of the device.
FIG. 1 illustrates a light emitting device including a semiconductor structure that includes a light-emitting active region 130A between an n-type layer 120 of III-V nitride semiconductor and a p-type layer 140 of III-V nitride semiconductor. A p-electrode 160 comprising silver metal is deposited on the p-type layer, and an n-electrode (not shown in FIG. 1) is coupled with the n-type layer. Means are provided by which electrical signals can be applied across said electrodes to cause light emission from the active region, and a migration barrier 175 is provided for preventing electrochemical migration of silver metal from the p-electrode toward the active region. The migration barrier 175 is a conducting guard sheet. The guard sheet encompasses the silver thoroughly, covering the edges of the silver p-electrode, as illustrated in FIG. 1.